Field of the Invention
This invention generally relates to strain measuring instruments or extensiometers, which generate an electrical signal output, and more particularly, to such instruments used to measure loads and forces in load carrying structures.
Electrical strain sensors have been known for many years and are being used to detect large stresses and overloads in cables, machine structures, frames of hydraulic presses and the like. Attempts have also been made to use such instruments in weighing applications, where the contents of large containers or silos are being monitored by measuring strains at certain locations in the supporting structure. However, such attempts have always failed because the strain levels in such structures are usually very low, in the order of a few tens of micro-strains. Existing strain sensors either do not have sufficient sensitivity to obtain useful results or the signal output is too much affected by extraneous factors like temperature and temperatue gradients, electrical noise and ground loops.
Errors caused by such influences can be considerably reduced by using a perfectly symmetrical arrangement of the electrical sensing elements and by connecting them in a balanced bridge circuit where one element is subjected to tension, the other to compression. The effects of outside disturbances then cancel each other out, while the effects to be measured are adding up. Furthermore, in order to minimize temperature effects, the two balanced strain sensitive resistors in the bridge circuit should be located in very close proximity to each other. They will then be subjected to equal temperatures even in cases where the outside temperature is changing rapidly and large temperature gradients do exist.
The design according to the invention achieves such nearly perfect symmetry by arranging the two sensing elements or strain gages in a central location on both sides of a slender metallic beam and electrically connecting them in a balanced bridge circuit. No appreciable temperature differences can develop across such a beam and temperature gradients along the length of the beam will affect both strain gages equally. On the other hand, any bending of the beam will put one strain gage into compression and the other strain gage into tension and will change their resistances differentially, thus causing an imbalance in the bridge circuit and generating an electrical signal. Even bending of the beam over its entire length is achieved by exerting two equal and opposite torques or couples at its ends. This is achieved by arranging two symmetrically but unevenly spaced pairs of flexible bridges near the ends of the beam which connect the beam to the two end sections of the instrument. Any motion of the one end section towards or away from the other will induce a rotational motion at the ends of the beam and cause it to evenly bend over its entire length.
In order to obtain a highly sensitive instrument, the strain gages used will preferably be of the conventional semiconductor type. These strain gages consist of fine fibers cut from a single silicon crystal. Such semiconductor strain gages have gage factors which are 50 to 80 times higher than those of metal foil gages, resulting in much higher sensitivity of the instrument. However, semiconductor strain gages are also highly temperature sensitive. A temperature change in the order of 20.degree. F. can result in a resistance change of about 10%, which does amount to the resistance change obtained under full load. It is obvious, therefore, that the two active elements in a semiconductor circuit have to be kept at exactly the same temperature. This can be best achieved by locating those strain gages in closest proximity, separated only by a thin layer of material of high thermal conductivity, like a metal for instance. This is the case in the design of this invention where the two strain gages are located exactly opposing each other at the two sides of a slender beam. The two gages will also have to be made of exactly the same material, have the same physical dimensions and the same electrical properties.